1. Field of the Invention
This specification relates to a method for fabricating graphene sheets or graphene particles, and particularly, to a graphene fabrication method using a supercritical condition.
2. Background of the Invention
Graphene is a single atomic layer of a honeycomb lattice, which is composed of carbon atoms that form hexagonal rings. Graphene is considered two-dimensional because it is one atom thick. Graphene is a rapidly rising star on various fields by virtue of excellent properties such as extremely high electrical mobility, thermal conductivity, mechanical strength, transparency quantized according to thickness, high specific surface and the like (Park et al., Nature Nanotechnology, 2010, 4, 217-224; Geim, Science, 2009, 324, 1530-1534; Allen et al., Chemical Reviews, 2010, 110, 132-145). The graphene may act as next generation energy storage materials, silicon substitutes, supercapacitors, lightweight components, electromagnetic shielding materials, sensors, displays and the like, and thus be applied to various fields of vehicle, energy, marine, aerospace, architecture, electronic product, medicine, military science and communication (Stankovich et al., Nature, 2006, 442, 282-286; Stoller et al., Nano Letters, 2008, 8, 3498-3502; Dikin et al., Nature, 2006, 448, 457-460; Ramanathan et al., Nature Nanotechnology, 2008, 3, 327-331; Blake et al., Nano Letters, 2008, 8, 1704-1708; Bunch et al., Science, 2007, 315, 490-493).
In order to apply the graphene to more various fields, a graphene fabricating method, which allows a mass production, an economical efficiency and a fast and simplified fabrication, need to be developed. Graphene fabrication methods, which have been introduced so far, include a chemical vapor deposition, a method for peeling a graphene single layer off from a graphite multi-layer sheet using an adhesive tape (“Scotch-tape” or “Peel off” method), a method for cutting carbon nanotubes, a thermal exfoliation for graphite, a chemical reduction and the like. Among others, the chemical reduction has many advantages, as compared with other graphene fabrication methods, of allowing a mass production, a relatively high economical efficiency and an introduction of various chemical functional groups (Kaner et al., Science, 2008, 320, 1170-1171). In addition, a graphene sheet, which is easily dispersible in an appropriate medium, is produced to be applicable to various fields, such as paper structures, thin film coating on various substrates, polymer nanocomposites and the like.
Upon employing the chemical reduction for graphene, graphite is oxidized (oxygenated) using an oxidizer to produce graphene oxide flake. The graphene oxide flake is a single atomic layer. Also, the graphene oxide flake has hydrophilicity due to a functional group, such as epoxy group (—O—), carboxyl group (—COOH), carbonyl group (—C═O), hydroxyl group (—OH) and the like, which are generated upon oxygenation, thereby being highly dispersible in polar solvents such as water or alcohols. However, the graphene oxide does not have the graphene-exclusive hexagonal structure due to the oxygenated functional group, and thereby rarely has an electrical conductivity. Consequently, the graphene oxide needs to be converted into graphene having high electrical conductivity through an appropriate deoxygenation or reduction.
Various chemical methods have been attempted to fabricate graphene through the deoxygenation or reduction with respect to the graphene oxide. Among others, a method using a strong reductant, such as hydrazine (NH2NH2), dimethylhydrazine (CH3NHNHCH3), hydroquinone (HOC6H4OH), sodium borohydride (NaBH4), hydrogen sulfide (H2S) and the like, has been introduced (Tung et al., Nature Nanotechnology, 2008, 4, 25-29; Lomeda et al., Journal of American Chemical Society, 2008, 130, 16201-16206; Stankovich et al., Nature, 2006, 442, 282-286; Wang et al., Journal of Physical Chemistry C, 2008, 112, 8192-8195; Si et al., Nano Letters, 2008, 8, 1679-1682; Hofman et al., Kolloid-Zeitschrift, 1934, 68, 149-151).
The use of strong reductant allows a relatively effective removal of oxygen, which results in fabrication of graphene having a relatively high electrical conductivity. However, most of strong reductants are very highly corrosive and explosive, very harmful to human bodies and causes environmental pollution. Accordingly, when fabricating the graphene in large quantity, a fabricating cost increases. When using hydrazine, which is well known as the most effective reductant for the deoxygenation of graphene oxide, the thusly-generated graphene contains nitrogen, which makes the graphene have an electrical conductivity much lower than that of graphite. Recognizing such problem, a method using a relatively weak reductant, such as sugar, vitamin C and the like, has been proposed (Zhu et al, ACS Nano, 2010, 4, 2429-2437; Gao et al, Chemistry of Materials, 2010, 22, 2213-2218). The use of weak reductant is eco-friendly and harmless to human bodies but oxygen is not effectively removed from the graphene oxide due to low reducing power of the reductant. Furthermore, when using the weak reductants, a graphite-exclusive hexagonal structure is not exhibited and accordingly defective graphene is fabricated, thereby lowering quality of the graphene due to the low electrical conductivity. In addition, a relatively long reaction time of 6 to 24 hours is required for removing more oxygen from the graphene oxide dispersed in an aqueous solution using the weak reductants and a batch type reaction is employed, thereby lowering uniformity and productivity of product.
Thus, to extensively apply graphene to various fields, the deoxygenation of the graphene oxide should be carried out more effectively to fabricate graphene with high quality and also the deoxygenation should be more eco-friendly and harmless to human bodies so as to reduce an additional fabricating cost for processing by-products. Accordingly, development of a method satisfying such requirements is urgently required. Also, it is required to develop a graphene fabricating method, in which the deoxygenation of the graphene oxide is rapidly carried out.